Polymer miscibility is inherently unpredictable. That is, for any given set of polymers, the underlying assumption has traditionally been that an immiscible blend will result. Thus, to discover sets of polymers that form miscible blends is typically viewed as being the exception, and not the rule.
The present invention therefore relates to miscible blends of two or more polyimides from a class of polyimide polymers having repeating units derived from certain dianhydride and diamine comonomers. According to the present invention, it has been found that miscible blends of such polyimide polymers will result even though the polyimide polymers utilized in the blend have structurally different dianhydride-derived moieties as well as structurally different diamine-derived moieties.
The miscible blends of the present invention may be usefully employed in end use applications requiring the physical properties of polyimides (e.g., as fabricated parts used in high temperature environments). In addition, the polyimide blends of this invention may be usefully employed to form membranes (e.g., in fiber or film form) used to copending U.S. Pat. application Ser. No. 505,099, now U.S. Pat. No. 5,055,116 filed in the names of Rachel S. KOHN et. al., on Apr. 3, 1990, and entitled "Gas Separation Membranes Comprising Miscible Blends of Polyimide Polymers", the entire content of which is expressly incorporated herein by reference.